1. Field
The present invention relates to apatite and a method for producing the same, and an apatite base material.
2. Description of the Related Art
It has been known that a semiconductor material such as titanium oxide has a photocatalytic function. Namely, when a semiconductor material such as titanium oxide absorbs light energy of a wavelength corresponding to a bandgap between a valence band and a conduction band, electrons in the valence band move to the conduction band by excitation, and positive electric charge (positive hole) is generated in the valence band. In a case where a foreign substance (such as organics) is adsorbed on the surface of the semiconductor material, electrons moving to the conduction band move to the organics on the surface of the semiconductor material and reduce the organics. In the valence band, the positive hole generated there seizes the electrons and oxidizes the organics. Particularly, since the positive hole in the titanium oxide has an extremely strong oxidizing power, it will decompose the organics into water and carbon dioxide ultimately. In recent years, the photocatalytic function (oxidation-decomposition function) of the titanium oxide has been used. Specifically, titanium oxide has been used for an antibacterial agent, a disinfectant, a deodorant, an environmental clean-up antipollution agent, and the like. However, since the titanium oxide itself has no capability of adsorbing organics onto the surface, available oxidation-decomposition function is limited.
Calcium hydroxyapatite Ca10(PO4)6(OH)2 as a main component of biological hard tissues such as teeth and bones will exchange ions easily with various cations and anions, and thus it has high bio-compatibility and absorptivity. Therefore, applications of the apatite to medical materials such as artificial bones and artificial dental roots, and to various fields such as chromatography adsorbents, chemical sensors, ion exchangers and catalysts, have been studied keenly. The apatite has a particularly remarkable capability in adsorbing organics such as protein.
In recent studies and development, a semiconductor material such as the above-mentioned titanium oxide and a compound based on calcium phosphate such as calcium hydroxyapatite are combined to obtain the features of both materials effectively (see for example, JP 2003-80078 A and JP 2003-321313 A).
Furthermore, calcium titanium hydroxyapatite Ca9Ti(PO4)6(OH)2 has been developed as well (see for example, JP 2000-327315 A, JP 2001-302220 A, JP 2003-175338 A and JP 2003-334883 A). This material is provided with a photocatalytic function by exchanging a part of calcium ions in the apatite with titanium ions. Thereby, this material has a photocatalytic function substantially equal to that of titanium oxide, and the efficiency of the photocatalytic function can be improved further due to the specific absorptivity of the apatite.
However, light energy required for exciting the photocatalyst having a strong oxidizing power (such as titanium oxide) is 3.2 eV, which corresponds to a light wavelength of about 380 nm. Therefore, titanium oxide can be excited with near-ultraviolet light but cannot be excited with visible light. Since the content of the UV light in sunlight is 4-5%, the photocatalytic function of the titanium oxide cannot be sufficient. In particular, the photocatalytic function cannot be exhibited indoors under fluorescent light including substantially no UV light.
Development of photocatalyst that can act even under visible light representing about 45% of sunlight has been desired strongly. Such a photocatalyst that acts under visible light will improve the efficiency remarkably and it can exhibit the photocatalytic function even indoors under fluorescent light.